1. Field of the Invention
The present invention relates to a camera system. More particularly, the present invention relates to a camera system where a compact varifocal lens system is applicable.
2. Description of the Prior Arts
Conventionally, four-component zoom lens system having a zoom ratio of approximately 6.times. has often been employed for an optical system of a video camera. Generally, such zoom lens system comprises a front lens component having a positive refractive power which is moved out in a focusing operation, a variator lens component having a negative refractive power, a compensator lens component having a negative or a positive refractive power and a master lens component.
Since, recently, a demand has been increasing for compact zoom lens systems for a video camera, zoom lens systems have been also proposed where a focusing operation is performed by an inner or a rear lens instead of component by a front lens component. For example, in U.S. Pat. No. 4,824,226, a focusing is operation performed by a compensator lens component, and in Japanese laid-open Patent Applications No. S59-28120 and No. S59-28121, by the rearmost lens element in a master lens component. For the zoom lens systems, a front lens component is employable which has a diameter smaller than that of a zoom lens system where a focusing operation is performed by the front lens component. A total length and weight of a zoom lens system decrease as the diameter of the front lens component decreases.
However, a satisfactory compactness is not obtainable since it is required to provide enough space on the front or the rear side of a focusing lens component to secure a sufficient shift amount for focusing.
Another zoom lens system has been also proposed which comprises a fixed front lens component having a positive refractive power, a variator lens component having a negative refractive power, a fixed lens component having a positive refractive power, and a compensator and focusing lens component having a positive refractive power. For example, in U.S. Pat. No. 4,818,083, a zoom lens system is proposed in which the compensator lens component in the conventional four-component zoom lens system is omitted and a rear lens unit of the master lens component serves as a compensator and focusing lens unit. Said rear lens unit of the master lens component, serving as a compensator and focusing lens unit, is located to have a relatively large space from a lens unit located at the object side of the rear lens unit, for being shiftable in both of the focusing operation and the zooming operation. In this zoom lens system, the number of lens elements can be reduced by omitting the compensator lens component. Also, no additional space is required for the focusing operation since the relatively large space in the master lens component is utilized therefor. Thus, the total length of this zoom lens system and the weight thereof can be decreased in comparison with the conventional four-component zoom lens system.
However, since the rear lens unit of the master lens component, serving as a compensator and focusing lens unit, consists of three or four lens elements in this zoom lens system, it is comparatively heavy as a movable lens unit (the weight of said compensator and focusing lens unit is twice or more times the weight of the compensator lens component in the conventional four-component zoom lens system). Therefore, the power required for driving said compensator and focusing lens unit is relatively in large in comparison with that required for driving the compensator lens component in the conventional four-component zoom lens system. Thus, this zoom lens system having said compensator and focusing lens unit is not suitable for the lens system in a video camera in which a focusing lens component must be driven at a high speed for a quick automatic focusing operation.